1. Field of the Invention
The invention relates to a suspended microstrip circuit comprising two parallel metal planes, a substrate arranged parallel thereto and therebetween and a first strip conductor on a first major surface of the substrate.
2. Description of the Prior Art
Such a suspended microstrip circuit is disclosed in the article by Dr. H. E. Brenner, "Use a computer to design suspended-substrate IC"s, Microwaves, September 1968, pages 38-46. By means of, inter alia, microstrip lines, microwave circuits such as filters, attenuators, T-junctions, mixers, circulators etc. can be made for, inter alia, radar and communication purposes.
Usually, such a microwave circuit is disposed in a fully closed conducting box. This box serves as a return path for the currents in the circuit; it shields the circuit from radiation from the environment and prevents radiation from the microwave circuit to the environment. The conducting box constitutes a length of "waveguide" which is short-circuited at both ends. The width of this "waveguide" is chosen so that no mode can propagate in it at the working frequency of the microwave circuit. This means that the "waveguide" must be rather narrow. For a microwave circuit of average size, it is therefore usually necessary to arrange the circuit in a plurality of separate, conducting boxes. In addition, this "waveguide" is difficult to realize at higher frequencies.
To obviate these drawbacks, it has already been proposed to use a wide "waveguide" which, in order to attenuate the modes which may occur, is provided with attenuation layers, the drawback then being that this entails considerable losses.
It is an object of the invention to provide a suspended microstrip circuit of the type defined in the opening paragraph which mitigates said drawbacks and inhibits in a simple manner the excitation and propagation of unwanted modes.